Terra Nova Working Interest Redetermination - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011		Dec. 31, 2010
Commitments and Contingencies Disclosure [Line Items]
Joint agreement ownership interest	12.00%			10.475%
Redetermination of Terra Nova working interest	$ 0	$ (5,351)	[1],[2]	$ 18,582	[1],[2]

[1] Reclassified to conform to current presentation.

[2] Results exclude corporate overhead, interest and discontinued operations.